Protein kinases constitute a large family of structurally related enzymes that are responsible for the control of a variety of signal transduction processes within the cell (see Hardie, G. and Hanks, S. The Protein Kinase Facts Book, I and II, Academic Press, San Diego, Calif.: 1995).
In general, protein kinases mediate intracellular signaling by effecting a phosphoryl transfer from a nucleoside triphosphate to a protein acceptor that is involved in a signaling pathway. These phosphorylation events act as molecular on/off switches that can modulate or regulate the target protein biological function. These phosphorylation events are ultimately triggered in response to a variety of extracellular and other stimuli. Examples of such stimuli include environmental and chemical stress signals (e.g. shock, heat shock, ultraviolet radiation, bacterial endotoxin, and H2O2), cytokines (e.g. interleukin-1 (IL-1) and tumor necrosis factor alpha (TNF-α), and growth factors (e.g. granulocyte macrophage-colony stimulating factor (GM-CSF), and fibroblast growth factor (FGF)). An extracellular stimulus may affect one or more cellular responses related to cell growth, migration, differentiation, secretion of hormones, activation of transcription factors, muscle contraction, glucose metabolism, control of protein synthesis, survival and regulation of the cell cycle.
Kinases may be categorized into families by the substrates they phosphorylate (e.g. protein-tyrosine, protein-serine/threonine, lipids etc). Sequence motifs have been identified that generally correspond to each of these kinase families (See, for example, Hanks, S. K., Hunter, T., FASEB J. 1995, 9, 576-596; Knighton et al., Science 1991, 253, 407-414; Hiles et al, Cell 1992, 70, 419-429; Kunz et al, Cell 1993, 73, 585-596; Garcia-Bustos et al, EMBO J 1994, 13, 2352-2361).
A serine/threonine kinase, protein kinase C-theta (PKC-theta), is a member of the novel, calcium independent PKC subfamily that is selectively expressed in T cells and skeletal muscle. Several lines of evidence indicate that PKC-theta has an essential role in T cell activation. Upon antigen stimulation of T cells, PKC-theta, but not other PKC isoforms, rapidly translocates from the cytoplasm to the site of cell contact between the T cell and antigen-presenting cell (APC), where it localizes with the T cell receptor (TCR) in a region termed the central supramolecular activation cluster (cSMAC) (Monks et al., 1997, Nature, 385: 83-86; Monks et al., 1998, Nature, 395: 82-86).
It has been reported that PKC-theta selectively activates the transcription factors AP-1 and NF-κB and integrates TCR and CD28 co-stimulatory signals leading to the activation of the CD28 response element (CD28RE) in the IL-2 promotor (Baier-Bitterlich et al., 1996, Mol. Cell. Biol., 16: 1842-1850; Coudronniere et al., 2000, PNAS, 97: 3394-3399). The specific role for PKC-theta in CD3/CD28 co-stimulation of T cells is highlighted in a study where expression of a kinase-dead PKC-theta mutant, or anti-sense PKC-theta dose-dependently inhibited CD3/CD28 co-stimulated NF-κB activation, but not TNF-alpha-stimulated NF-κB activation. This was not seen with other PKC isoforms (Lin et al., 2000, Mol. Cell. Biol., 20: 2933-2940). Recruitment of PKC-theta to the SMAC is reported to be mediated by its N-terminal regulatory domain and is necessary for T cell activation, as an over-expressed PKC-theta catalytic fragment did not translocate and was unable to activate NF-κB, whereas a PKC-theta catalytic domain-Lck membrane-binding domain chimera was able to reconstitute signaling (Bi et al., 2001, Nat. Immunol., 2:556-563).
Translocation of PKC-theta to the SMAC appears to be mediated by a largely PLC-gamma/DAG-independent mechanism, involving Vav and PI3-kinase (Villalba et al., 2002, JCB 157: 253-263), whilst activation of PKC-theta requires input from several signaling components including Lck, ZAP-70, SLP-76, PLC-gamma, Vav and PI3-kinase (Liu et al., 2000, JBC, 275: 3606-3609; Herndon et al., 2001, J. Immunol., 166: 5654-5664; Dienz et al., 2002, J. Immunol., 169: 365-372; Bauer et al., 2001 JBC., 276: 31627-31634). These biochemical studies in human T cells have gained credence from studies in PKC-theta knockout mice, which have confirmed a crucial role for this enzyme in T cell function. PKC-theta−/− mice are healthy and fertile, have a normally developed immune system, but exhibit profound defects in mature T cell activation (Sun et al., 200, Nature, 404:402-407). Proliferative responses to TCR and TCR/CD28 co-stimulation were inhibited (>90%) as were in vivo responses to antigen. In agreement with studies on human T cells, activation of the transcription factors AP-1 and NF-κB was abrogated, resulting in a severe deficit in IL-2 production and IL-2 R upregulation (Baier-Bitterlich et al., 1996, MBC, 16, 1842; Lin et al., 2000, MCB, 20, 2933; Courdonniere, 2000, 97, 3394). More recently, studies in PKC-theta-deficient mice have indicated a role for PKC-theta in the development of mouse models of autoimmune diseases, including multiple sclerosis (MS), rheumatoid arthritis (RA) and irritable bowel disease (IBD) (Salek-Ardakani et al., 2006; Tan et al., 2006; Healy et al., 2006; Anderson et al., 2006). In these models, PKC-theta-deficient mice exhibited a marked reduction in disease severity that was associated with a profound defect in the development and effector function of autoreactive T cells.
In addition to its role in T cell activation, PKC-theta is reported to mediate the phorbol ester-triggered survival signal that protects T cells from Fas- and UV-induced apoptosis (Villalba et al., 2001, J. Immunol. 166: 5955-5963; Berttolotto et al., 2000, 275: 37246-37250). This pro-survival role is of interest because the human PKC-theta gene has been mapped to chromosome 10 (10p15), a region associated with mutations leading to T cell leukaemias and lymphomas (Erdel et al., 1995, Genomics 25: 295-297; Verma et al., 1987, J. Cancer Res. Clin. Oncol., 113: 192-196).
Another serine/threonine protein kinase, Glycogen synthase kinase-3 (GSK-3), is a comprised of α and β isoforms that are each encoded by distinct genes [Coghlan et al., Chemistry & Biology 2000, 7, 793-803; and Kim and Kimmel, Curr. Opinion Genetics Dev., 2000 10, 508-514]. GSK-3 has been implicated in various diseases, disorders, and conditions including diabetes, Alzheimer's disease, CNS diseases such as bipolar disorder, schizophrenia, cerebral stroke, Huntington's and other neurodegenerative diseases, leukocytopenia and cardiomyocyte hypertrophy [PCT Application Nos.: WO 99/65897 and WO 00/38675; and Haq et al., J. Cell Biol. 2000, 151, 117-130]. These diseases, disorders, and conditions are associated with the abnormal operation of certain cell signaling pathways in which GSK-3 plays a role. GSK-3 has been found to phosphorylate and modulate the activity of a number of regulatory proteins. These proteins include glycogen synthase, which is the rate limiting enzyme necessary for glycogen synthesis, the microtubule associated protein Tau, the gene transcription factor β-catenin, the translation initiation factor e1F2B, as well as ATP citrate lyase, axin, heat shock factor-1, c-Jun, c-myc, c-myb, CREB, and CEPBα. These diverse protein targets implicate GSK-3 in many aspects of cellular metabolism, proliferation, differentiation, and development.
In a GSK-3 mediated pathway that is relevant for the treatment of type II diabetes, insulin-induced signaling leads to cellular glucose uptake and glycogen synthesis. Along this pathway, GSK-3 is a negative regulator of the insulin-induced signal. Normally, the presence of insulin causes inhibition of GSK-3 mediated phosphorylation and deactivation of glycogen synthase. The inhibition of GSK-3 leads to increased glycogen synthesis and glucose uptake [Klein et al., PNAS 1996, 93, 8455-8459; Cross et al., Biochem. J. 1994, 303, 21-26); Cohen, Biochem. Soc. Trans. 1993, 21, 555-567; and Massillon et al., Biochem J. 1994, 299, 123-128]. However, in a diabetic patient, where the insulin response is impaired, glycogen synthesis and glucose uptake fail to increase despite the presence of relatively high blood levels of insulin. This leads to abnormally high blood levels of glucose with acute and long-term effects that may ultimately result in cardiovascular disease, renal failure and blindness. In such patients, the normal insulin-induced inhibition of GSK-3 fails to occur. It has also been reported that in patients with type II diabetes, GSK-3 is overexpressed [see, PCT Application: WO 00/38675]. Therapeutic inhibitors of GSK-3 are therefore potentially useful for treating diabetic patients suffering from an impaired response to insulin.
GSK-3 activity is associated with Alzheimer's disease. The hallmarks of this disease are the extracellular plaques formed by aggregated $3 amyloid peptides and the formation of intracellular neurofibrillary tangles via the tau protein.
It has been shown that GSK-3 inhibition reduces amyloid-β peptides in an animal model of Alzheimer's disease. See pages 435, 438. Phiel et. al., Nature 423, 435-439 (2003). Mice over-expressing amyloid precursor protein (APP) treated with lithium (a GSK-3α inhibitor) over a three-week period showed over a 50% decrease in amyloid-β peptide tissue levels.
The neurofibrillary tangles contain hyperphosphorylated Tau protein, in which Tau is phosphorylated on abnormal sites. GSK-3 is known to phosphorylate these abnormal sites in cell and animal models. Conditional transgenic mice that over-express GSK-3 develop aspects of AD including tau hyperphosphorylation, neuronal apoptosis and spatial learning deficit. Turning off GSK-3 in these mice restores normal behavior, reduces Tau hyperphosphorylation and neuronal apoptosis. (Engel T et al., J Neuro Sci, 2006, 26, 5083-5090 and Lucas et al, EMBO J, 2001, 20, 27-39) Inhibitors of GSK-3 have also been shown to prevent hyperphosphorylation of Tau in cells [Lovestone et al., Current Biology 1994, 4, 1077-86; and Brownlees et al., Neuroreport 1997, 8, 3251-55].
GSK-3 as a target for psychosis and mood disorders, such as schizophrenia and bipolar disease, respectively, have been reported in the literature. AKT haplotype deficiency was identified in a subset of schizophrenic patients which correlated with increased GSK-3 activity. A single allele knockout of GSK-3β resulted in attenuated hyperactivity in response to amphetamine in a behavior model of mania.
Several antipsychotic drugs and mood stabilizers used to treat both schizophrenic and bipolar patients have been shown to inhibit GSK-3 (Emamian et al, Nat Genet, 2004, 36, 131-137; Obrien et al, J Neurosci, 2004, 24, 6791-6798; Beaulieu et al, PNAS, 2004, 101, 5099-5104; Li et al Int J Neuropsychopharmacol, 2006, pp 1-13; Gould T D, Expert Opin Ther Targets, 2006, 10, 377-392). Furthermore, a recent patent, US 2004/0039007 describes GSK 3 inhibitors that show anti-schizophrenic and anxiolytic effects in relevant mouse behavior models.
GSK-3 activity is associated with stroke. Wang et al. showed that IGF-1 (insulin growth factor-1), a known GSK 3 inhibitor, reduced infarct size in rat brains after transient middle cerebral artery occlusion (MCAO), a model for stroke in rats. [Wang et al., Brain Res 2000, 859, 381-5; Sasaki et al., Neurol Res 2001, 23, 588-92; Hashimoto et al., J. Biol. Chem. 2002, 277, 32985-32991]. US 2004/0039007 describes the effect of GSK 3 inhibitors in MCAO, a stroke model in rats. These GSK 3 inhibitors significantly reduced striatal ischemic damage and reduce edema formation in rats. Additionally, the rats “demonstrated marked improvement in neurological function over the time course of the experiment.”
Accordingly, there is a great need to develop compounds useful as inhibitors of protein kinases. In particular, it would be desirable to develop compounds that are useful as inhibitors of GSK-3 and PKC-theta, particularly given the inadequate treatments currently available for the majority of the disorders implicated in their activation.